452 THE GEOLOGY OF MINNESOTA. 



[Granite. 



No. 619. GRANITE. (Poriilnjritic, granophyric.) 



Pigeon point. Prom the same hill as the last, but from the south side where the structure is basaltic. 

 Compare No. 1845. 



Ref. Annual Report, x, page 59. 



Red, granitic rock, somewhat porphyritic with feldspar and with quartz. 



Mir. The quartz permeates the feldspars in a micrographic manner, which is 

 sometimes spherulitic. It also forms many grains which are sometimes aggregated 

 like the grains of a fragmental quartzyte. Quartz phenocrysts of much larger size 

 also appear, forming bipyramidal crystals like those of a quartz-porphyry. Occa- 

 sionally such phenocrysts show embayments into which the molten rock entered. 

 In one such c'ase is seen a beautiful illustration and demonstration of the very early 

 origin of the granophyric and pseudo-spherulitic structure. This is illustrated by 

 figure 11 of plate I. The embayment was filled with the original (molten?) magma, 

 and on consolidation this has also become crystallized and has assumed an imperfectly 

 granophyric structure. This is continuously traceable through the neck of the 

 embayment and into the surrounding rock mass, and in the latter the same structure 

 is developed on a more perfect scale. It is impossible but that the material filling 

 the embayment and that surrounding the phenocryst were in the same molten 

 condition at the same time. They must have assumed simultaneously these low 

 crystalline structures at once on consolidation, or very soon thereafter. If the 

 consolidated matter were at first glassy, and this structure were then developed as a 

 result of devitrification it might be considered a secondary structure, but it was 

 probably not glassy, since in the near vicinity of this quartz phenocryst, and generally 

 distributed through the section, are other minerals whose outlines are confusedly 

 linked in with the outlines of the granophyric figures. These are hiotitc and an 

 occasional indistinct form of feldspar. T*here are also many quartzes, as already 

 mentioned, which must date from the second consolidation. The section illustrates 

 one of the steps in the transition from a quartz-porphyry to a granite, being not 

 fairly referred to either. One section. 



Age. Acid Cabotian. 



Remark. In the foregoing it is assumed, as is usual, that the existence of 

 bipyramidal quartz crystals is demonstrative of the former molten condition of the 

 quartz-porphyry in which they occur; and that is perhaps assuming too much, 

 inasmuch as such crystals are known to arise in the aqueous transformation (with 

 heat) of quartzytes of Pigeon point in portions adjacent to the great dikes, and in 

 which there is no possibility of a former molten condition, since the most of the 

 rock retains its rounded clastic grains. This rock may therefore have resulted from 

 a clastic which was transformed by hot solutions, but was not molten, and hence 

 the quartz phenocrysts may be of secondary origin. N. H. w. 



